This invention relates to sensitive instruments and, more particularly, to a fluid responsive instrument that can be used as a pressure sensor, a flowmeter, or a micrometer among other applications.
In a hot wire anemometer, the hot wire is connected to serve as one arm of an electrical bridge circuit. Current passing through the hot wire heats the wire, thereby increasing its resistance. The hot wire is disposed in an elongated cavity through which the gas to be measured flows and cools the hot wire accordingly. If the type of gas passing through the cavity is known, the resistance change of the hot wire is a measure of the gas flow rate. If the flow rate of the gas passing through the cavity is known, the resistance change of the hot wire is a measure of the thermal conductivity of the gas and, hence, the gas type.
My U.S. Pat. No. 3,971,247, which issued July 27, 1976, discloses a thin elongated hot wire bent in half to extend along the length of a cavity formed in a housing. The ends of the hot wire are soldered to pads on a printed circuit board located at one end of the cavity for support and electrical connection to a bridge circuit. The middle of the hot wire is wrapped around a rod at the other end of the cavity for support. The rod is deformed to exert tension on the hot wire as its length changes. Thus, for a cavity having a given length, the length of the hot wire can be doubled and a corresponding increase in sensitivity can be achieved. But, the probability of a short circuit by contact between halves of the hot wire or the hot wire and the sides of the cavity rises, as the length of the cavity increases. This type of flowmeter is capable of responding rapidly to changes in gas flow, i.e., responding to a change from zero to peak voltage in the order of a hundred milliseconds. However, to generate signal voltages greater than 500 millivolts, a gas flow of 1.5 liters per minute, or greater, past the hot wire is required. Such large gas flow introduces turbulence, which increases the probability of a short circuit of the hot wire and a noisy signal.